The present invention relates to an interdigital transducer for surface wave arrangements having busbars, transducer fingers alternately connected to the busbars, and finger gaps between the transducer fingers.
Surface wave arrangements operating by means of acoustic waves which propagate in the surface of a substrate have been known for approximately two decades. Such arrangements are used as electronic filters instead of arrangements comprising inductors and capacitors. The interdigital transducers to be used as output or input transducers comprise a number of finger-shaped electrode strips which are arranged on the substrate surface in an interdigital assignment relative to one another. To be more precise, these fingers are (electrically) connected in an alternating fashion to one or the other busbar.
In order to have a filter with a prescribed filter curve, it has been known just as long to select the overlap lengths of the interdigital fingers of the transducer in a manner matched to the filter curve. This is thus the so-called overlap weighting.
Examples of overlap weighting are described in EP-A-0,188,263 and DE-A-2,546,193. Both examples have, moreover, a design such that the transducers are reflection-free. The finger widths and the finger gaps are positioned in a fundamentally lambda-periodic fashion. Individual offsets are required for the freedom from reflection.
A surface wave structure which can also be overlap-weighted is described in DE-A-4,010,310, which was not published prior to the date of filing the present application. In this structure, final fingers are provided which, with respect to their finger width and their finger spacing, have different dimensions by comparison with the other fingers of the structure. These divergently dimensioned fingers are not weighted and serve merely to suppress reflections which otherwise emanate from final fingers.
Also known is finger-omission weighting, the desired weighting of the transducer being realized by purposive omission of individual fingers of the transducer. Omitted fingers can be replaced in this case by non-alternating electrically connected fingers. Moreover, finger-offset weighting is known, in which individual prescribed fingers of a transducer are arranged offset with respect to the prescribed periodicity of the transducer. These fingers are therefore no longer exactly in phase with the transducer periodicity of the comparatively un-weighted interdigital transducer.
An example of a finger-omission weighting is described in EP-A-0,369,835. FIG. 7 of this printed publication shows an interdigital finger structure having fingers whose centerline spacings are non-periodic. The finger centerline spacing is 4.5 lambda at the ends of the transducer and 1 lambda at the center of the transducer, and spacings of other fingers have values between these multiples of the wavelength lambda. There are no special instructions for the finger gaps.
These types of weighting which have been mentioned above have been adequately described before, together with their advantages and disadvantages, in the prior art. Overlap weighting leads to problems in adhering to the aperture of a transducer. Many of the characteristics of the overlap-weighted transducer can be described only in two dimensions. In particular, the influence of manufacturing tolerances becomes strongly noticeable in the case of only small overlap lengths. This results frequently in problems in designing filters and in the reproducibility at critical points. Furthermore, it is only with reluctance that two overlap-weighted transducers are used directly opposite one another in a filter. To be precise, this is attended by the difficulty of having to correlate each individual excitation in the transducer operating as input transducer with each detecting overlap of the transducer operating as output transducer. It is known, for example in television band pass filters, to weight only one transducer and to provide the second transducer with a time signal which corresponds to a simple rectangle.
The method of finger-omission weighting, which is more rarely applied, in any case, has the disadvantage that the desired time signal cannot be effectively scanned, because all that is available for each overlap is to assign "ones" and "zeros", that is to say finger present and finger omitted, so that the dynamics of this weighting method are mostly unsatisfactory.